Mid Sweden University

It becomes more and more common to print tag antennas using electrically conductive ink for mass-produced Radio Frequency IDentification (RFID) tags. Electrical properties of the ink are mostly determined by conductive (e.g. silver) particles mixed into the ink solution. Since silver is relatively expensive it is desirable to minimize the amount of ink used per antenna. This paper illustrates how the printed conductor area of the antenna can be reduced by applying a grid pattern to an existing antenna geometry and to what extent the gridding can be performed without significantly degrading of the antenna electrical properties. Two common antenna structures are used as an example. It is also shown that by slightly modifying the original antenna geometry it is possible to even further reduce the amount of used ink.

Antennas in RFID tags have often been designed in a single layer with copper as conductor and plastic foils as substrate. There is currently a large interest in roll to roll production of RFID tags and silver based inks have been developed for use in printed RFID antennas. Silver ink based single layer antennas works well and is providing 70% to 80% of the reading range compared to copper solutions. However, more advanced antennas are needed to provide less sensitivity to the environment of RFID tags .i.e. need for placing tags on metal or near water. In this work we present a study of multilayered antennas so called patch antennas, for 2.45 GHz RFID tags. The advantage of the patch antenna is that it can be applied to any kind of material, reflecting or lossy material, and still provide good antenna function. However, the patch antenna efficiency is strongly dependent on the material used. For low cost RFID tags in logistics there is a need to manufacture the antenna as a part of the packaging process. In the current work we have investigated the possibility to manufacture printed patch antennas of common packaging materials.

In some of the radio-frequency identification (RFID) applications, RFID tag antennas will be printed onto flexible substrates. Sticked onto different surfaces, geometric distortion of the tag antenna, such as bending of the tag, could occur due to antenna substrate flexibility. In the present paper performance degradation due to such distortion of an RFID system, using the common half-wave dipole antennas usually used in tags, is simulated. It is shown that degradation of RFID system performance in terms of operating range due to such distortion is significant and that antenna structures less sensitive for this deformation should be considered for RFID systems where this kind of effect may occur.

To remotely identify a metallic object with an RFID tag it is required that the tag’s antenna consists of more than one layer of conducting material, where one layer can comprise the metallic object itself, or alternatively one can use the object itself to make a slot antenna. Tag antennas are also seldom smaller than about ¼ of a wavelength. This paper presents a capacitively fed PIFA for passive UHF RFID wherethe radiating element has received elliptical geometry and is placed inside a circular metallic cup that constitutes the tagantenna’s boundary conditions and is protected by a top layer ofan extremely hard plastic material that also works as a superstrate. The elliptic radiating patch has length of only 0.068 wavelengths and with its cup it has total antenna diameter of0.084 wavelengths and total antenna height 0.016 wavelengths.The final RFID tag can be placed upon a metallic or non-metallicsurface or immersed into the same to create an overall flat surface, as long as it is optimized for the specific situation.

The paper presents a concept where pairs of ordinary RFID tags are exploited for use as remotely read moisture sensors. The pair of tags is incorporated into one label where one of the tags is embedded in a moisture absorbent material and the other is left open. In a humid environment the moisture concentration is higher in the absorbent material than the surrounding environment which causes degradation to the embedded tag’s antenna in terms of dielectric losses and change of input impedance. The level of relative humidity or the amount of water in the absorbent material is determined for a passive RFID system by comparing the difference in RFID reader output power required to power up respectively the open and embedded tag. It is similarly shown how the backscattered signal strength of a semi-active RFID system is proportional to the relative humidity and amount of water in the absorbent material. Typical applications include moisture detection in buildings, especially from leaking water pipe connections hidden beyond walls. Presented solution has a cost comparable to ordinary RFID tags, and the passive system also has infinite life time since no internal power supply is needed. The concept is characterized for two commercial RFID systems, one passive operating at 868 MHz and one semi-active operating at 2.45 GHz.